Skylarkify default_platform_defs macros.
Skylarkify default_platform_defs macros.

Reviewed By: adamjernst

Differential Revision: D6527119

fbshipit-source-id: 1e31b9491d2cf501f348d544482c025096fd81b1

facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/yoga